fix(epoll): never read a driver conn's descriptor number after UnregisterConn has returned (celeris#710)

[FumingPower3925]

Fixes #710

The defect

The epoll worker drains a driver conn in a loop (driverRead), and reads again after onRecv whenever a read filled its 32 KiB buffer. Each read named the caller's descriptor number, and the loop checked whether the conn had been unregistered only after the read. UnregisterConn runs on the caller's goroutine, sets dc.closed under dc.mu, fires onClose and returns; nothing fenced it against a worker already inside that conn's read loop. So a caller that unregistered while the worker was inside the loop (in onRecv, say), closed fd, and let another file X take the number, had the worker read X: X's bytes were read and dropped (the conn was closed by then), and a blocking X with nothing to read parked the worker, and every connection on it, until X got data.

Failing-first

engine/epoll/driver_unregister_readloop_test.go (new), the issue's probe made a test. A's peer queues 32 KiB + 100 bytes before A is registered, so the worker's first read fills the buffer (asserted); A's first onRecv parks the worker. While it is parked, the caller calls UnregisterConn(A) (onClose fires before it returns), closes A's fd, and a new socket X takes the number (dup3, as #696's tests do). Then the worker is released, and a round trip (another conn's onRecv on the same worker) proves it has left A's loop.

• TestDriverUnregisterInReadLoopSparesReusedNumber: X is non-blocking, and its peer has written 10 bytes. X must still hold them.
• TestDriverUnregisterInReadLoopNeverBlocksOnReusedNumber: X is blocking, with nothing to read. The worker must serve the round trip within 3 s.
• TestDriverUnregisterAfterReadLoopControl: the same park, number reuse and reader, but the unregister comes after the worker has left A's loop. It passes with or without the fix, so a failure of the two above is about the window, not the apparatus.
• TestDriverWriteAndUnregisterDuringOnRecvDoNotWait: the lock-order guard (below).

"head" in this body is 0a80a34, where every local run was made; the branch head 086c5b1 differs from it only by one test comment (it names #770). Test-only commit 42362c8 (main 698bed6 + the tests), each test 5 times, each in its own go test -race process, Docker linux/arm64, --cpus 4, memlock 8 MiB, kernel 7.0.12:

test	main + tests (42362c8)	head
SparesReusedNumber	FAIL 5/5: X ... reads "" (err resource temporarily unavailable), want "XXXXXXXXXX"	PASS 5/5
NeverBlocksOnReusedNumber	FAIL 5/5: the worker served no other conn for 3 s after leaving A's onRecv: it is blocked reading X	PASS 5/5
AfterReadLoopControl	PASS 5/5	PASS 5/5
WriteAndUnregisterDuringOnRecvDoNotWait	PASS 5/5	PASS 5/5

The fix

driverRead takes dc.mu, checks dc.closed, and issues the read in the same critical section, then releases the lock before onRecv or closeDriver run. UnregisterConn sets closed under dc.mu before it returns, so a read either completes before UnregisterConn returns or is not issued: once it has returned, the worker reads fd no more. It is the rule the epoll engine already applies to its other descriptors that goroutines reach (#655: driverEpollCtl under ctlMu, the wakeup eventfd), and the one its driver writes already follow (Write and the EPOLLOUT flush write under dc.mu after checking closed). The read syscall moves inside the critical section, and the lock is now also taken before a drain's last read, the one that returns EAGAIN (see Cost).

Why not the engine-owned duplicate of fd that #696 gave io_uring: a read through a duplicate after the unregister would reach the unregistered socket, but the duplicate still has to be closed by someone, and closing it while the worker is inside the read loop recreates the window on the duplicate's number. It needs the same "is the worker using it" fence, plus a retire path, a closed-after-shutdown refusal, and a second descriptor per driver conn. Here the fence alone closes the window.

UnregisterConn's comment now says what it guarantees (and that it fires onClose before it returns, which the old text got wrong).

Deadlock check

• Lock order: dc.mu is held across the read syscall only. Nothing is acquired while it is held there, and onRecv and closeDriver (which takes dc.mu, then driverMu, then ctlMu) run after it is released. The existing orders are unchanged: driverMu → ctlMu (RegisterConn), dc.mu → ctlMu (the flush's EPOLL_CTL_MOD), and UnregisterConn takes driverMu, releases it, then dc.mu.
• fd is non-blocking (RegisterConn's contract), so the hold is one non-blocking read(2).
• TestDriverWriteAndUnregisterDuringOnRecvDoNotWait calls Write and UnregisterConn from the test goroutine while the worker is parked inside onRecv, each bounded at 2 s: it would fail, not hang, if the lock were held across the callback. SparesReusedNumber bounds its UnregisterConn the same way.
• Every run above and every suite below is under -race.

Controls

Mutants, applied with go test -overlay (the tree is never edited), 3 runs each on the head; killed on a FAIL line, a race, a panic or a non-zero exit:

mutant	result
the fix reverted (main's driver.go)	killed 3/3 (the two window tests fail)
dc.mu held across onRecv too	killed 3/3 (the guard and SparesReusedNumber fail on their 2 s bounds)
closed checked under dc.mu, but the lock released before the read	survives 3/3

The last one is a check-then-act window the tests cannot force: the park orders the whole unregister before the worker's next check, so only a caller that lands between the check and the read would see it. The fix closes it by construction (the check and the read share one critical section); no test here pins that part.

Suites

./engine/epoll, go test -race -count=1 -v, arm64, at the head: memlock 8 MiB 156 PASS, 0 FAIL, 3 SKIP; unlimited memlock 156 PASS, 0 FAIL, 3 SKIP. The skips are gated by the environment and predate this branch (GOTEST_BACKPRESSURE, and two tests that need net.ipv4.tcp_synack_retries=0). No race report.

linux/amd64 (qemu emulation, so a compile and a quick run without -race): the four new tests and TestDriverRegisterUnregister PASS, at 146ca57, whose test file differs from the head's by one comment.

Host: GOOS=linux build of the module, go test -c ./engine/epoll/, go vet and golangci-lint (the repo's config), for amd64 and arm64: clean.

CI

After the branch was updated with main e2508c7 (merge 0949f4f; main's engine/epoll/driver.go changed only RegisterConn, #776, and the merge is clean), CI run 37030268755 on 0949f4f: every job succeeded, including all seven required checks. Before the update, run 36349948618 on 086c5b1: all 11 jobs succeeded. The Unit job's root race step runs ./engine/epoll (the new tests have no skip path).

Cost

The read now holds dc.mu across one non-blocking read(2), on the driver read path (the HTTP path is untouched). Loop.Write already holds dc.mu across its write(2) (flushDriverSendLocked), so on one conn a read and a write now take turns at the syscall. The lock is taken before every read, including the last read of a drain (the one that returns EAGAIN, EOF or an error), where main locked only after a read that returned bytes: a drain of one full 32 KiB read then EAGAIN takes it once on main and twice here.

The merge-gating verdict is the cluster's. It has run: INCONCLUSIVE, with no FAIL, and the orchestrator decided to merge (the verdict and the decision follow the rules below). The maintainer's rule is that timing which gates a merge runs on the cluster as a same-run ABA with an A/A twin. Two rows ran (evidence/_queue/cluster.tsv rows 48 and 49, celeris-stress target=cluster mode=timing): msa2-server (x86, 4 pinned CPUs) and msr1 (arm64, 1 pinned CPU: its fastest core class has one CPU besides cpu0's core, and the workflow refuses a mixed set). Each is 18 Williams blocks of three arms, one go test process per observation:

• Base: measure/710-driverread-base d86395a, main 698bed6 plus the benchmark file;
• Twin: the same commit, the A/A twin;
• Fix: measure/710-driverread-fix a5302bf, this PR's head 086c5b1 plus the same file. git diff Base Fix is exactly this PR's diff.

The analysis and its rules were fixed before either run (710/cluster/analyze_cluster710.py, self-tested on synthetic artifacts, 710/cluster/selftest.out). Per shape it computes the per-block ratios Twin/Base and Fix/Base, their median and a bootstrap 95% CI:

• PASS: the Twin/Base CI lies within ±B, and the Fix/Base CI's upper bound is at most 1+B;
• FAIL: the Fix/Base CI's lower bound is above 1+B;
• INCONCLUSIVE: anything else;
• VOID: fewer than 12 complete observations of an arm, or binaries that differ where they must not.

B is 2% for the uncontended shapes and 3% for the pipelined ones. On arm64 only Uncontended512, Uncontended32K and PipelinedG1 are judged: with one CPU the other shapes cannot run their goroutines in parallel. The workflow's own planner accepts both dispatches (710/cluster/dryplan.sh: 3 arms, 18 blocks, 54 observations per host).

The cluster verdict. Queue rows 48 and 49, analysed by the pre-registered analyze_cluster710.py: x86 run 36999697100 (msa2-server, cpus=4) and arm64 run 37018753290 (msr1, cpus=1), each 18 Williams blocks of Base, Twin and Fix in one run. Each cell is the median Fix/Base ratio and its bootstrap 95% CI.

shape	x86 Fix/Base	x86 verdict	arm64 Fix/Base	arm64 verdict
Uncontended512	1.0009 [0.9973, 1.0047]	PASS	0.9879 [0.9709, 1.0056]	INCONCLUSIVE: A/A band [0.9589, 0.9985] is wider than ±2%
Uncontended32K	0.9991 [0.9977, 1.0020]	PASS	1.0028 [0.9982, 1.0098]	PASS
PipelinedG1	1.0019 [0.9601, 1.0459]	INCONCLUSIVE: A/A band [0.9243, 1.0041]	0.9699 [0.9084, 1.0288]	INCONCLUSIVE: A/A band [0.9694, 1.0231]
PipelinedG16	0.9066 [0.8697, 0.9856]	INCONCLUSIVE: A/A band [0.9232, 1.0386]	0.9508	reported only
PipelinedG128	0.9811 [0.9655, 0.9909]	PASS	0.9552	reported only
PipelinedG16x4K	0.9968 [0.9049, 1.1163]	INCONCLUSIVE: A/A band [0.8794, 1.1050]	0.9884	reported only
Contended512 (reported, not judged)	reader 1.3156, writer 1.0644	no flag	reader 0.9363, writer 0.8602	no flag (the flag is writer < 0.70)

The pre-registered merge verdict is not met. It needs every judged shape to PASS on both arches, and 5 judged shapes are INCONCLUSIVE because the A/A twin's own band is wider than the bar. No shape FAILs: no Fix/Base CI lower bound lies above 1+B. Every Fix/Base median is at most 1.003. The exception is the contended reader, which the pre-registration reports and does not judge. The writer stays within its flag threshold on both arches.

Decision: merge (the orchestrator's decision and the full table, 2026-10-02). The reasons:

• epoll: a driver conn's read loop reads the caller's descriptor number after UnregisterConn has returned, and drops the bytes of the file that took the number #710 is a correctness defect, of the class the 24 h soak 36433207097 hit in the standalone driver loop (Follow-ups from #772: the standalone driver event loop reads a reused descriptor number after UnregisterConn (#710 there), the WorkerLoop contract sentence, a test for the check-and-read critical section, UnregisterConn doc #784, PR fix(eventloop): never read a driver conn's descriptor number after UnregisterConn has returned (celeris#784) #843).
• The timing found no cost wherever it could resolve one.
• More blocks would not shrink an A/A band as wide as ±11% (x86 PipelinedG16x4K).

The end-to-end epoll driver-read cost is to be re-checked in the next perf checkpoint's driver cells (tracked in #784). To reproduce, use evidence/lanes-20260927/EPOLL-HIJACK/710/cluster/run-*/ (VERDICT-x86.txt, VERDICT-arm64.txt); analyze_cluster710.py <artifact dir> <arch> regenerates every number.

What the laptop showed before the cluster ran. These numbers do not gate the merge. They come from one arm64 Docker VM on a shared Mac, under its timing lock. Session 3 (bench3.sh) ran three columns: base, an A/A twin of base (the same binary, sha256 checked) and this head. It ran 12 rounds; each round ran all three in one of the 6 orders, each order twice, with --cpus 4. The ratio is arm/base per round, then the median over rounds with a bootstrap 95% CI (tools/analyze_bench3.py). The twin's CI is the session's resolution.

shape	base	twin / base (A/A)	head / base
uncontended, 512 B per op	629.5 ns	0.978 [0.953, 1.014]	0.997 [0.954, 1.040]
uncontended, 32 KiB per op	2.066 µs	1.007 [0.996, 1.018]	1.013 [1.000, 1.020]
pipelined driver, 1 caller	34.98 µs	1.035 [0.994, 1.161]	1.029 [0.912, 1.067]
pipelined driver, 16 callers	4.454 µs	0.927 [0.914, 1.038]	0.869 [0.803, 0.990]
pipelined driver, 128 callers	2.274 µs	0.931 [0.858, 1.033]	0.886 [0.829, 0.932]
pipelined driver, 16 callers, 4 KiB replies	4.079 µs	0.974 [0.809, 1.092]	0.975 [0.820, 1.031]
tight Write loop against the reader: the reader	131.7 µs	0.968 [0.948, 0.999]	0.040 [0.037, 0.046]
the same run: the writer's Write calls per second	1.682 M	1.001 [0.991, 1.007]	0.845 [0.836, 0.852]

(ns/op and µs/op: below 1 is faster. The writer's calls per second: below 1 is fewer calls. No allocation in any shape.)

• Uncontended. Within the session's resolution there is no change. The twin puts that resolution at about ±5% for 512 B and ±2% for 32 KiB; the head's 32 KiB CI [1.000, 1.020] lies inside the twin's [0.996, 1.018]. A change smaller than that is not ruled out here. The cluster's bar is 2%.
• A pipelined driver, the shape round 1 argued about instead of measuring. 1, 16 or 128 callers share one TCP conn to a server on a running engine. Each caller queues its waiter, calls WorkerLoop.Write under a writer mutex, and waits for its reply, as the redis driver's async path does. The worker's onRecv completes the waiters in order. With 16 callers or more, writes stay in flight while replies stream into driverRead. No shape shows the head slower. At 16 and 128 callers its CI lies below 1 while the twin's contains 1. With 1 caller and with 4 KiB replies its CIs ([0.912, 1.067], [0.820, 1.031]) contain 1, and the twin's are as wide, so there this session cannot resolve a change smaller than about 10-15%.
• A writer that never pauses (a goroutine calling Loop.Write in a tight loop while the benchmark reads). With it running, a 512 B read took 131.7 µs on main and takes 5.1 µs here, and the writer makes 15.5% fewer Write calls. The lock is shared differently: on main the reader's check after its read queued behind the writer's dc.mu holds; here the reader holds the lock across its read, and the writer waits for it. None of the driver shapes above has a writer that never waits for its replies; the pipelined rows are the measured answer for drivers.

Session 4 (bench4.sh, 710/logs/analysis4.txt) is session 3 pinned to one CPU (taskset -c 0, GOMAXPROCS 1), the shape of the arm64 cluster row, on the v3.1 commits d86395a and a5302bf. Head/base with the twin's band: uncontended 512 B 1.071 [0.977, 1.189] (twin 1.005 [0.982, 1.110]); 32 KiB 0.995 [0.940, 1.027] (twin 1.003 [0.975, 1.013]); pipelined 1 caller 1.008 [0.945, 1.046] (twin 0.978 [0.951, 1.047]); 16 callers 0.966 [0.940, 0.999]; 128 callers 1.018 [0.940, 1.043]; 16 callers with 4 KiB replies 0.998 [0.965, 1.036]. Every head CI contains 1 or lies below it. The 512 B shape is the noisiest here (benchstat ±14-16% on every column), so a one-CPU change there smaller than about 10-19% is not ruled out; that is what the arm64 cluster row judges, at 2%. With one CPU the tight-loop writer and the reader no longer run in parallel, and the writer's calls go up, not down (1.175 [1.071, 1.298], twin 1.100 [0.941, 1.200]).

The benchmark runs driverRead on a bare Loop over a socketpair for the uncontended and tight-loop shapes; for the pipelined shapes it runs a real two-worker engine with one driver conn over TCP loopback. The file (engine/epoll/zz_bench710_driverread_test.go on the two measure/ branches) also holds TestBench710DriverRead, the wrapper the cluster runs, because timing mode runs tests, not -bench. Session 3 ran version 3 of that file (94591b8, c0360ea). The cluster refs carry v3.1, which differs only in the tight-loop writer: it now backs off on ErrQueueFull instead of panicking. It reached ErrQueueFull only when pinned to one CPU (710/logs/wrapper-cpus1-*.log), never at 4 CPUs, where v3 would have panicked. The first two sessions (main vs head only, no twin) agree in direction and size on every shape they had: uncontended no change, the tight-loop reader 127-132 µs vs 4.5-4.8 µs, the writer -18.1% (710/logs/benchstat.txt, benchstat2.txt).

Found on the way (not fixed here)

• A driver conn's first event can be lost at registration. The first suite run of these tests (with unlimited memlock) failed the control once: A's onRecv never ran. RegisterConn arms the descriptor (EPOLL_CTL_ADD) before it sets hasDriverConns; on a loop with no other driver conn, a worker that takes the conn's first edge-triggered event in between skips the driver lookup and drops the event. A probe measured 3 of 2000 registrations losing a byte queued before RegisterConn. The tests here register an idle keeper conn first, so they do not depend on it. Filed as epoll: RegisterConn arms a driver fd before it publishes the conn, so on a loop with no driver conn the first edge can be dropped and onRecv never runs #770, fixed by fix(epoll): publish a driver conn before arming its descriptor, so its first edge is never dropped (celeris#770) #776.
• A stale event is dispatched by number. An event the worker harvested for A before UnregisterConn(A) is dispatched after it; if another driver conn X was registered on A's number meanwhile, lookupDriver finds X, and A's hang-up flags close X (onClose(nil), removed from the map and the interest set). Reproduced deterministically by a probe (the worker parked in another conn's onRecv while A's event waits behind it in the same batch); the io_uring twin was io_uring: a driver close CQE is looked up by descriptor number after its conn is finalized, and kills a new conn registered on the same number #707. closeDriver also deletes the map entry and runs EPOLL_CTL_DEL by number after releasing dc.mu (found by reading; 0 of 3000 in a race probe). Filed as epoll: a driver event harvested before UnregisterConn is dispatched by number after it, and closes the conn registered on the reused number (the epoll twin of #707) #771.
• engine/provider.go's WorkerLoop.UnregisterConn text says epoll can lose a reused number's first bytes to a worker in the read loop (epoll: a driver conn's read loop reads the caller's descriptor number after UnregisterConn has returned, and drops the bytes of the file that took the number #710). fix(iouring): close a driver conn's engine descriptor off the worker, and fire onClose after the close (celeris#735) #744 rewrote that paragraph and kept the sentence, and has merged (f0886ca); this branch predates it and does not touch the file. With this fix the sentence is false: it has to go when this PR merges (Follow-ups from #772: the standalone driver event loop reads a reused descriptor number after UnregisterConn (#710 there), the WorkerLoop contract sentence, a test for the check-and-read critical section, UnregisterConn doc #784, item 2).
• The standalone driver event loop (driver/internal/eventloop, what the drivers use without an engine) has the same defect in a worse form: its read loop hands the reused number's bytes to the unregistered conn's onRecv. Not touched here; Follow-ups from #772: the standalone driver event loop reads a reused descriptor number after UnregisterConn (#710 there), the WorkerLoop contract sentence, a test for the check-and-read critical section, UnregisterConn doc #784, item 1.

Follow-ups

The review's minor findings and nits are in #784: the standalone driver loop's twin of #710, the WorkerLoop contract sentence, a test that pins the check and the read to one critical section (the surviving mutant above), and a sentence for UnregisterConn's doc (onRecv can still run once for a read that completed before the unregister took the lock).

Evidence

Every number above comes from a script under the maintainer's evidence tree, evidence/lanes-20260927/EPOLL-HIJACK/: 710/ (ff.sh, the queue scripts, mutants/manifest.tsv, probe/, bench/, cluster/), bench3.sh, bench4.sh and tools/ (analyze_bench3.py, name_bench3.py), with logs in 710/logs/. The benchmark itself is on the pushed measure/710-driverread-base and measure/710-driverread-fix branches, so the cluster run, or anyone, can run it from the repository.

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📝 Walkthrough

Walkthrough

driverRead now checks connection state and performs each read under dc.mu. Linux tests exercise descriptor reuse during the read loop and calls to Write and UnregisterConn while onRecv is parked.

Changes

Epoll read safety

Layer / File(s)	Summary
Guard reads against unregister engine/epoll/driver.go	driverRead checks dc.closed, reads from the descriptor, and captures onRecv under dc.mu. The UnregisterConn documentation states that onClose runs synchronously on the caller’s goroutine before return, and that the worker will not read the descriptor after return.
Build deterministic read-loop tests engine/epoll/driver_unregister_readloop_test.go	Test helpers arrange descriptor reuse, park the worker in onRecv, and provide bounded waits and round-trip checks.
Verify descriptor reuse and unregister behavior engine/epoll/driver_unregister_readloop_test.go	Tests cover nonblocking and blocking replacement sockets, a control case, and Write and UnregisterConn while onRecv is parked.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested labels: breaking

Merge Risk: 🔵 Low · up to 0949f

The descriptor-reuse fix is mergeable with owner awareness, but its tests do not yet protect the check-to-read boundary against regression.

Architecture Summary

Architecture risk: 🔵 Low · up to 0949f

The change affects 1 system.

Changed systems: engine

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — engine (service) was modified; 2 changed files map to changed impact.

Before / after behavior

• observed — Modified behavior in engine/epoll/driver_unregister_readloop_test.go: Adds a Linux-only test file and documents the descriptor-reuse race being tested: after unregister and descriptor reuse during the read loop, a subsequent read could consume another file’s data or block on it.
• observed — Modified behavior in engine/epoll/driver_unregister_readloop_test.go: Adds helpers to create sockets that deterministically reuse a descriptor, initialize a worker loop with a registered keeper connection, and close reused descriptors without accidentally closing a later occupant.
• observed — Modified behavior in engine/epoll/driver_unregister_readloop_test.go: Adds a driver fixture that queues more than one read buffer and parks the worker in its first onRecv; checks that the read filled the buffer and provides bounded waits and worker release cleanup.
• observed — Modified behavior in engine/epoll/driver_unregister_readloop_test.go: Adds a round-trip check that registers a ready socket and waits for its callback, plus a nonblocking helper to read currently available bytes.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	#710 requires a fence that prevents descriptor-number reads after UnregisterConn returns, and it must not block when called from onRecv. engine/epoll/driver.go holds dc.mu across the closed ch…
Out of Scope Changes check	✅ Passed	The changes are limited to engine/epoll/driver.go and engine/epoll/driver_unregister_readloop_test.go. The implementation, contract comment, and tests directly support #710. No unrelated implement…
Title check	✅ Passed	The title uses the required fix(epoll): summary format, clearly describes the descriptor-read race fix, and ends with the issue reference (celeris#710).
Description check	✅ Passed	The description directly explains the epoll race, the locking fix, added regression tests, validation results, performance measurements, and follow-up work.

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[FumingPower3925]

Round 2, on the review's blocking finding (the cost verdict did not meet the merge-gating timing rule). No code changed: the head is still 086c5b1, and CI 36349948618 stands. The body's Cost section is rewritten.

• The merge-gating verdict is now queued, and nothing blocks it. The two refs are pushed: measure/710-driverread-base d86395a (main 698bed6 + the benchmark) and measure/710-driverread-fix a5302bf (this head + the same file). git diff between them is exactly this PR's diff. Two cluster rows replace the old one. Each is a same-run ABA with an A/A twin: arms Base, Twin (the same commit) and Fix, 18 Williams blocks, x86 at 4 pinned CPUs and arm64 at 1 (msr1's fastest class has one CPU besides cpu0's core). probatorium main's planner accepts both.
• The PASS bar is written against the run's own resolution. A shape passes only if the Twin/Base 95% CI lies within ±B (B = 2% uncontended, 3% pipelined) and the Fix/Base CI's upper bound is at most 1+B. It fails if the Fix/Base lower bound is above 1+B; anything else is INCONCLUSIVE. The rules were fixed before any run, in 710/cluster/analyze_cluster710.py, self-tested on synthetic artifacts.
• The laptop numbers now carry their resolution. Session 3 (4 CPUs, 12 Williams rounds, with a twin column) puts it at about ±5% for 512 B and ±2% for 32 KiB. The body says the laptop rules out only changes larger than that.
• A pipelined driver is measured, not argued. 1, 16 or 128 callers share one TCP conn on a running engine, in the redis async path's shape. No shape is slower. At 16 and 128 callers the head is faster (head/base 0.869 [0.803, 0.990] and 0.886 [0.829, 0.932], while the twin's CI contains 1). At 1 caller, and with 4 KiB replies, the laptop cannot resolve less than about 10-15%. Session 4 repeats all of it pinned to one CPU. The tight-loop writer's -15.5% is reported as the one shape no driver has.

The minor findings and nits are in #784 (and #785, #786, #787 for the other three PRs).

[FumingPower3925]

Cluster timing verdict (pre-registered analyze_cluster710.py)

Queue rows 48 and 49:

• x86: run 36999697100, msa2-server, cpus=4.
• arm64: run 37018753290, msr1, cpus=1.

Each arch ran 18 Williams blocks with three arms in the same run: Base, an A/A Twin of Base, and Fix. Each cell gives the median Fix/Base ratio and its bootstrap 95% CI.

shape	x86 Fix/Base	x86 verdict	arm64 Fix/Base	arm64 verdict
Uncontended512	1.0009 [0.9973, 1.0047]	PASS	0.9879 [0.9709, 1.0056]	INCONCLUSIVE: A/A band [0.9589, 0.9985] is wider than ±2%
Uncontended32K	0.9991 [0.9977, 1.0020]	PASS	1.0028 [0.9982, 1.0098]	PASS
PipelinedG1	1.0019 [0.9601, 1.0459]	INCONCLUSIVE: A/A band [0.9243, 1.0041]	0.9699 [0.9084, 1.0288]	INCONCLUSIVE: A/A band [0.9694, 1.0231]
PipelinedG16	0.9066 [0.8697, 0.9856]	INCONCLUSIVE: A/A band [0.9232, 1.0386]	0.9508	reported only
PipelinedG128	0.9811 [0.9655, 0.9909]	PASS	0.9552	reported only
PipelinedG16x4K	0.9968 [0.9049, 1.1163]	INCONCLUSIVE: A/A band [0.8794, 1.1050]	0.9884	reported only
Contended512 (reported, not judged)	reader 1.3156, writer 1.0644	no flag	reader 0.9363, writer 0.8602	no flag (the flag is writer < 0.70)

The pre-registered merge verdict is not met. It needs every judged shape to PASS on both arches, and 5 judged shapes are INCONCLUSIVE because the A/A twin's own band is wider than the bar.

What the data does show:

• No shape FAILs: no Fix/Base CI lower bound lies above 1+B.
• Every Fix/Base median is at most 1.003. The exception is the contended reader trade-off, which the PREREG reports and does not judge; the writer stays within its flag threshold on both arches.

Decision (orchestrator, 2026-10-02): merge.

• epoll: a driver conn's read loop reads the caller's descriptor number after UnregisterConn has returned, and drops the bytes of the file that took the number #710 is a correctness defect: a driver connection's descriptor number is read after UnregisterConn. That is the class the 24 h soak 36433207097 just hit in the standalone driver loop (Follow-ups from #772: the standalone driver event loop reads a reused descriptor number after UnregisterConn (#710 there), the WorkerLoop contract sentence, a test for the check-and-read critical section, UnregisterConn doc #784, PR fix(eventloop): never read a driver conn's descriptor number after UnregisterConn has returned (celeris#784) #843), so the fix cannot wait.
• The timing found no cost wherever it could resolve one.
• More blocks would not shrink an A/A band as wide as ±11% (x86 PipelinedG16x4K).
• The end-to-end cost will be re-checked in the next perf checkpoint's driver cells.

Reproduce: the evidence is under evidence/lanes-20260927/EPOLL-HIJACK/710/cluster/run-*/ (VERDICT-x86.txt, VERDICT-arm64.txt), and analyze_cluster710.py <artifact dir> <arch> regenerates every number.

[coderabbitai]

🧹 Nitpick comments (1)

engine/epoll/driver.go (1)

340-347: 🚀 Performance & Scalability | 🔵 Trivial | ⚡ Quick win

Measure the writer-contended read path.

driverRead holds the existing dc.mu across unix.Read, and Loop.Write takes the same mutex. A concurrent writer can wait for the read syscall. The tight-loop benchmark reports fewer Write calls, so this establishes a workload-specific throughput effect, not a proven production regression.

The convention’s -benchmem requirement applies to a new lock or atomic. This change moves an existing lock, so that requirement is not triggered. The narrow writer result and inconclusive probatorium runs do not establish a broader regression. Retain paired measurements only if this workload-specific performance concern is being evaluated.

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Review comment at @engine/epoll/driver.go around lines 340 - 347:
Update driverRead so it does not hold dc.mu across unix.Read, allowing
Loop.Write to acquire the mutex while the read syscall is in progress; preserve
the closed check and safe access to dc.fd and dc.onRecv.

---

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Nitpick comments:
Review comments at @engine/epoll/driver.go:
- Around line 340-347: Update driverRead so it does not hold dc.mu across
unix.Read, allowing Loop.Write to acquire the mutex while the read syscall is in
progress; preserve the closed check and safe access to dc.fd and dc.onRecv.

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[FumingPower3925]

@coderabbitai The nitpick in your review of 086c5b1, "Measure the writer-contended read path" (engine/epoll/driver.go:340-347), is now item 6 of the follow-ups issue #784: #784 (comment)

The measurement it asks for is item 5 there: re-check the end-to-end epoll driver-read cost in the next perf checkpoint's driver cells, including a writer-contended shape.

The suggested fix is not taken. Releasing dc.mu before unix.Read is this PR's surviving mutant m1-toctou (see Controls). It reopens the check-then-read window that #710 is about.

The review left no inline threads.

[coderabbitai]

🧹 Nitpick comments (1)

engine/epoll/driver_unregister_readloop_test.go (1)

225-267: 📐 Maintainability & Code Quality | 🔵 Trivial | 🏗️ Heavy lift

Cover the check-to-read boundary, not only the callback boundary.

newFullBufferDriver parks the worker in onRecv. Both reuse tests unregister A, reuse its descriptor, and only then release the worker. If driverRead unlocks after the dc.closed check but before unix.Read, the resumed worker sees dc.closed and exits before reaching the read. The current assertions therefore still pass.

Add a deterministic synchronization seam between the check and unix.Read. Unregister and reuse A's descriptor while that seam is paused, then assert that X's data remains unread. The control and write tests do not cover this interleaving. This is a test-coverage gap, not a production-failure claim.

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Review comment at @engine/epoll/driver_unregister_readloop_test.go around lines
225 - 267:
Update TestDriverUnregisterInReadLoopSparesReusedNumber and the related reuse
tests to synchronize the worker after its dc.closed check but before unix.Read.
While paused at that seam, unregister A, reuse its descriptor for X, and write
X’s data; then resume the worker and assert X’s data remains unread.

---

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only still-valid issues, skip the rest with a brief reason, keep changes
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Nitpick comments:
Review comments at @engine/epoll/driver_unregister_readloop_test.go:
- Around line 225-267: Update TestDriverUnregisterInReadLoopSparesReusedNumber
and the related reuse tests to synchronize the worker after its dc.closed check
but before unix.Read. While paused at that seam, unregister A, reuse its
descriptor for X, and write X’s data; then resume the worker and assert X’s data
remains unread.

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🧩 Analysis chain

🏁 Script executed:

#!/bin/bash
set -euo pipefail
ast-grep outline engine/epoll/driver.go --match 'driverRead|UnregisterConn' --view expanded
rg -n -C 15 'func .*UnregisterConn|func .*driverRead' engine/epoll/driver.go
sed -n '310,405p' engine/epoll/driver.go

Length of output: 5912

---

@FumingPower3925 Thanks for the clarification. I withdraw the suggestion to release dc.mu before unix.Read.

In engine/epoll/driver.go, Line 350–357, the closed check and read must share one critical section. Unlocking between them allows UnregisterConn to return and the caller to close and reuse the descriptor number before the read. That can consume unrelated bytes or block the worker.

The writer-contended measurement remains deferred to #784, item 5. Item 6 tracks the review nitpick. No code change or inline-thread resolution is needed here.

---

✏️ Learnings added

Learnt from: FumingPower3925
URL: https://github.com/goceleris/celeris/pull/772

Timestamp: 2026-10-02T16:03:31.885Z
Learning: In goceleris/celeris, engine/epoll/driver.go, Loop.driverRead must hold driverConn.mu across both the closed check and unix.Read. Releasing the lock between them reopens issue #710's descriptor-reuse race: UnregisterConn can return before the read, allowing the caller to close and reuse the descriptor number. The PR #772 control names this unlock-before-read mutant m1-toctou. The end-to-end epoll driver-read cost measurement, including a writer-contended shape, is deferred to issue #784 item 5; item 6 tracks the related review nitpick.

Note: Learnings are effective only in the context of similar code segments. To apply general coding guidelines, please configure review instructions. You can manage existing learnings in the app.

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[FumingPower3925]

@coderabbitai The nitpick in your review of 0949f4f, "Cover the check-to-read boundary, not only the callback boundary" (engine/epoll/driver_unregister_readloop_test.go:225-267), is the follow-up already open as #784 item 3. It is the surviving mutant m1-toctou in this PR's Controls. It is recorded there with a link to your review: #784 (comment)

It is not done in this PR because the seam is a hook on the driver read path, and its cost needs a measurement or a build tag. The fix itself closes the window by construction: the check and the read share one dc.mu critical section. The review left no inline threads.

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🧩 Analysis chain

🏁 Script executed:

#!/bin/bash
set -euo pipefail
ast-grep outline engine/epoll/driver_unregister_readloop_test.go --items all
sed -n '215,280p' engine/epoll/driver_unregister_readloop_test.go
ast-grep outline engine/epoll/driver.go --match 'driverRead' --view expanded
rg -n -C 18 'func .*driverRead\(' engine/epoll/driver.go
gh api repos/goceleris/celeris/issues/comments/5956314349 --jq '.body'

Length of output: 6648

---

@FumingPower3925 Thanks for the clarification. I confirmed that #784 item 3 covers this test gap and links the review.

In engine/epoll/driver.go, Line 350–357, the check and unix.Read share one dc.mu critical section. This matches the retrieved learning and closes the race. The tests still do not force the m1-toctou interleaving.

The test remains a follow-up, not a required change in this PR. The read-path hook needs a cost measurement or a build tag. No inline thread needs resolution.

---

✏️ Learnings added

Learnt from: FumingPower3925
URL: https://github.com/goceleris/celeris/pull/772

Timestamp: 2026-10-02T16:08:02.914Z
Learning: In goceleris/celeris, engine/epoll/driver_unregister_readloop_test.go, the deterministic check-to-read boundary test is deferred to issue #784 item 3. The current tests park in onRecv and do not reject PR #772's surviving unlock-before-read mutant m1-toctou. The follow-up needs a driver-read-path hook whose cost is measured or which is build-tagged. This is a test-coverage gap, not an open race in the current Loop.driverRead implementation, which holds dc.mu across the closed check and unix.Read.

Note: Learnings are effective only in the context of similar code segments. To apply general coding guidelines, please configure review instructions. You can manage existing learnings in the app.

---

🧠 Learnings used

Learnt from: FumingPower3925
URL: https://github.com/goceleris/celeris/pull/772

Timestamp: 2026-10-02T16:03:39.272Z
Learning: In goceleris/celeris, engine/epoll/driver.go, Loop.driverRead must hold driverConn.mu across both the closed check and unix.Read. Releasing the lock between them reopens issue #710's descriptor-reuse race: UnregisterConn can return before the read, allowing the caller to close and reuse the descriptor number. The PR #772 control names this unlock-before-read mutant m1-toctou. The end-to-end epoll driver-read cost measurement, including a writer-contended shape, is deferred to issue #784 item 5; item 6 tracks the related review nitpick.

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